Electrical brake systems, generally referred to as “brake by wire” systems, are increasingly being integrated into or replacing conventional hydraulic brake systems. Such electrical brake systems are preferable because they reduce the mass of the system and provide greater ability to integrate the system into the vehicle's other electronic circuits and controls.
During depression of the brake pedal in a conventional hydraulic braking system, the hydraulic fluid will exert a force back on the brake pedal due to the hydraulic pressure in the brake lines. Since an electronic brake system may not have such hydraulic pressure at the brake pedal, the vehicle operator will not detect any countering force, which in turn can disorient the operator. Accordingly, a typical electrical brake system will include a brake pedal feel simulator to provide a simulation force on the brake pedal. The simulation force provided by the simulator acts opposite the brake pedal force generated by the vehicle operator.
The Applicants have discovered a drawback to such brake pedal feel simulators. During emergency conditions or failure conditions, the brake pedal feel simulator continues to oppose the depression of the brake pedal by the operator and hence reduces the amount of force transmitted through the brake pedal. As used herein, emergency conditions are defined as situations where a large amount of braking force is required in a short period of time, and generally includes brake pedal forces greater than 200 N. As used herein, failure conditions are defined as a power failure or the failure of brake boosters to supplement the braking force, which also generally include high brake pedal forces greater 200 N. Thus, the simulator opposes the operation at times when high brake forces are needed.
Accordingly, there exists a need to provide a brake pedal feel simulator which automatically adjusts its operation to reduce or eliminate the simulation force during emergency or failure conditions.